Numerous portable devices have been developed in which optical sensors are used to detect, measure, and display various physiological parameter information of a user. For example, some devices detect and measure the variation in blood flow through arteries or blood volume in subcutaneous tissue. Applications for such optical sensors include the monitoring of heart rate, glucose level, apnea, respiratory stress, and other physiological conditions. The optical sensor of such arrangements include one or more light sources that illuminate a targeted portion of the human body and one or more associated optical detectors that receive a portion of the optical energy emitted by the light sources. There are two basic types of such arrangements. In transmissive sensor arrangements, a relatively thin portion of the body such as the tip of the finger or the earlobe is positioned between a light source and a photo detector. Light that passes through the body tissue impinges on the photo detector resulting in an electrical signal that is synchronized to each heartbeat. In reflective sensor arrangements, a sensor that includes one or more light sources located in spaced apart juxtaposition with a photo detector is positioned against a targeted area of the body. Optical energy emitted by the light sources passes through the skin of the targeted tissue region, is scattered, partially absorbed, and is reflected by the body (e.g., blood flowing through arteries and other vascular structure). In some applications, the reflected optical energy is in effect modulated in accordance with blood flow in the targeted area and detected by the photo detector. The detected reflection can then be used to produce a signal pulse that is synchronized to each heartbeat. In both transmissive and reflective arrangements, the signal produced by the photo detectors is processed to display or otherwise provide a real time indication of the monitored physiological parameter.
One area of growing interest in the use of physiological monitors is with respect to personal wellness and/or physical exercise for purposes of fitness training and weight loss. Technological advances relating to optical sensors, signal processing, and display devices have made it possible to realize small, light-weight physiological monitors that can be embodied as armbands or bracelets that are comfortably worn by a user. For example, the embodiments described herein comprises an optical sensor that may be included in a wearable device.
Providing physiological monitors for wellness and physical exercise applications is subject to numerous design and manufacturing considerations. For example, the electronic circuitry for processing the signal produced by the photo detector and displaying an indication of the monitored parameter must operate at a low power level to provide adequate battery life while simultaneously providing sufficient accuracy. Constraints relating to the physical design of such monitors are not limited to the challenges of packaging the electronics and display units in an arrangement that can be easily and comfortably worn by a user. Special considerations and constraints are present with respect to incorporation of the optical sensor. For example, the light sources and photodiode of the optical sensor must be optically isolated from one another. Otherwise, the photo detector will receive optical energy that is not modulated by heartbeat, which can result in an unwarranted increase in electrical design requirements and/or seriously affect monitoring accuracy and power requirements. Similarly, optimal performance requires that the optical sensor be firmly positioned against the user's skin so that light emitted by an optical source passes through the skin and, additionally, so that ambient light does not reach an associated photo detector. Firmly positioning the optical sensor against the user's skin also is important with respect to preventing movement of the sensor that can affect the accuracy of the monitoring device and/or interrupt its operation. Additionally, the optical sensor should be securely retained by the monitoring device to maintain physical integrity and facilitate satisfactory waterproofing of the entire monitor.
Because of the above mentioned design and manufacturing considerations, as well as others that are known to designers and manufacturers, a need exists for improved systems and techniques for incorporating optical sensor arrangements in physiological monitoring devices. The need is of special significance relative to personal wellness, activity, sleep, and exercise monitors. In particular, the manufacturing costs of such devices must be maintained as low as possible to provide a generally affordable and competitive product without sacrificing product accuracy and quality.